An advantageous method for automatically initiating processing or sorting operations on articles traveling along a conveyor is to apply retro-reflective code marks on the articles and "read" those marks with a photoscanner. The photoscanner includes a light source that sends out one or more beams of light and a photoelectric cell adjacent each beam of light that develops an electric signal whose magnitude depends on the amount of light that strikes the cell. Since retro-reflective code marks return light substantially along the line that light struck the marks, the registration of a retro-reflective code mark with a beam transmitted by the photoscanner will cause a signal of large magnitude to be developed in the cell, and the photoscanner is set so that such a signal will trigger the processing or sorting operation.
Full exploitation of photoscanning to automate processing and sorting operations has been prevented by limitations in the past methods for applying retro-reflective code marks. Retro-reflective liquid coating compositions, which comprise a liquid vehicle and transparent glass spherical beads dispersed in the vehicle as taught in U.S. Pat. Nos. 2,963,378 and 3,228,897, offer the best adaptability to automatic application, but the existing varieties of such coating compositions have other disadvantages. One major disadvantage of the prior coating compositions arises from the way in which the compositions become retro-reflective: upon application of the compositions, the glass beads are distributed as a thin layer and the vehicle recedes until the exterior beads partially protrude above the main portion of the coating and are optically exposed, though covered with a thin transparent skin of vehicle. This process takes a comparatively long time, and, as a result, marks formed from existing compositions by desired methods of application do not become sufficiently retro-reflective to trigger a photoscanner until several minutes after application.
Such a several-minute delay is intolerable in many potential applications, since it is typically desirable for a processing or sorting station on an automated line to follow the marking station quite closely. In fact, most often, articles traveling on an automated line should reach the processing or sorting station a few seconds after the marking station. Since no retro-reflective liquid coating composition has been practicably useful to form marks that are reflective in just a few seconds, such coating compositions have not been used to form retro-reflective code marks on a large percentage of lines where they could otherwise be used to automate the line.